Exiled Cambodian opposition figurehead Sam Rainsy accused the country's powerful former leader Hun Sen of ordering the killing of Lim Kimya on Tuesday.
Lim Kimya was shot dead by a gunman on a motorbike as he arrived in the Thai capital from the Cambodian city of Siem Reap by bus, accompanied by his French wife.
Police have issued a warrant for the suspected shooter, who was not named, with local media reporting that the wanted man is a former Thai marine.
"We are investigating the motives. As of now, we don't have much information apart from the fact that he works as a motorcycle taxi driver," area police chief Sanong Sangmanee told AFP.
Scores of Cambodian opposition activists have fled to Thailand in recent years to avoid alleged repression at home. Some were arrested and deported back to the country.
Hun Sen ruled Cambodia with an iron fist for nearly four decades, with rights groups accusing him of using the legal system to crush opposition to his rule.
He stepped down and handed power to his son Hun Manet in 2023 but is still seen as a major power in the kingdom.
Government accused
Sam Rainsy, Hun Sen's long-time rival, accused the former Cambodian leader of being behind the killing.
"Hun Sen's hand can be seen behind the assassination of Lim Kimya, just as it has been behind the countless political crimes in Cambodia that have always gone unpunished," Sam Rainsy said in a statement posted on his Facebook page.
He alleged "the motive is the same, as is the modus operandi".
Cambodian government spokesman Pen Bona told AFP the killing was in Thailand "so Thai authorities will handle the case".
He denied the government involved with the killing, saying opposition figures "always accuse the government of everything groundlessly and without any evidence".
Lim Kimya became an opposition member of Cambodia's parliament following an election in 2013 in which Hun Sen's ruling party almost lost to its then-rival, the Cambodia National Rescue Party (CNRP).
The CNRP, which was founded in 2012 by Sam Rainsy and Kem Sokha and was once considered the sole viable opponent to the ruling Cambodian People's Party (CPP), was dissolved by court order in 2017.
Scores of opposition politicians and MPs, including Lim Kimya, were banned from political activities following the party's dissolution.
The now-dissolved CNRP said in a statement it was shocked by "the heinous and inhumane" killing of Lim Kimya.
It condemned "the brutal act that badly threatens political freedom".
Bryony Lau, deputy Asia director at Human Rights Watch, called on Thai authorities to "promptly and thoroughly investigate and prosecute those responsible for the killing of Lim Kimya".
Lim Kimya also held French citizenship, but the French embassies in Bangkok and Phnom Penh have so far declined to comment on the killing.
